http://sarien.net/
This site has some of the original Sierra adventure games available to play for free in your browser. There's also one game that works on the iPad browser.
The games available for desktops/laptop browsers:
King's Quest: Quest for the Crown
Police Quest: In Pursuit of the Death Angel
Space Quest I: The Sarien Encounter
The Black Cauldron
You can play The Black Cauldron on your iPad